Levulinic acid is a well-known product of hexose acid hydrolysis, and is inexpensively obtained from cellulose feedstocks. Consequently, it is an attractive starting material in producing useful 5-carbon compounds, such as methyltetrahydrofuran and derivatives. 5-methyl-dihydro-furan-2-one, also known as gamma-valerolactone, can be produced from levulinic acid as shown below. 
U.S. Pat. No. 5,883,266 discloses the use of a bifunctional catalyst having a first function of hydrogenating and a second function of ring opening to prepare a variety of products from levulinic acid including 5-methyl-dihydro-furan-2-one.
U.S. Pat. No. 2,786,852 disclosed production of 5-methyl-dihydro-furan-2-one from levulinic acid using a reduced copper oxide catalyst.
U.S. Pat. No. 4,420,622 discloses preparation of 5-alkyl-butyrolactones from levulinic esters using metal catalysts.
The present method represents an advance in the art by offering a process that exploits several advantages of using a supercritical fluid (SCF) as the reaction solvent. SCFs are attractive media for conducting chemical transformations, primarily because the solvent and transport properties of a single solution can be varied appreciably and continuously with relatively minor changes in temperature or pressure. The density variation in a SCF also influences the chemical potential of solutes and thus reaction rates and equilibrium constants. Thus, the solvent environment can be optimized for a specific reaction application by tuning the various density-dependent fluid properties. For a discussion of advantages and applications of supercritical fluid media for chemistry and catalysis, see Hutchenson, K. W., “Organic Chemical Reactions and Catalysis in Supercritical Fluid Media,” in Supercritical Fluid Technology in Materials Science and Engineering, Y. -P. Sun (ed.), Marcel Dekker: New York (2002), pp. 87-187. SCF-mediated reaction processes have the potential for utilizing a reaction medium that exhibits improved safety, health, and environmental impact relative to typical organic solvents. Carbon dioxide and other SCF solvents are generally considered environmentally benign, nontoxic, nonflammable, and inexpensive, useful solvent at relatively moderate temperatures.